The present invention relates to flooring systems designed to reduce airborne and impact sound transmission, and more specifically relates to an improved acoustical flooring underlayment which improves acoustical isolation while avoiding cracking of the finished floor.
Conventional flooring systems include a subfloor of poured concrete or plywood. Various underlayments located between the subfloor and the finished floor (typically ceramic tile, vinyl tile or hardwood) have been used to reduce sound transmission. Sound rated or floating floor systems are known in the prior art for acoustically isolating a room beneath a floor on which impacts may occur, such as pedestrian footfalls, sports activities, dropping of toys, or scraping caused by moving furniture. Impact noise generation can generally be reduced by using thick carpeting, but where concrete, ceramic tile, sheet vinyl, or hardwood finishes are to be used, a sound rated floor may be particularly desirable. The transmission of impact noise to the area below can be reduced by resiliently supporting the floor away from the floor substructure, which typically transmits the noise into the area below. If the floor surface receiving the impact is isolated from the substructure, then the impact sound transmission will be greatly reduced. Likewise, if the ceiling below is isolated from the substructure, the impact sound will be restricted from traveling into the area below.
Sound rated floors are typically evaluated by ASTM Standard #492 and are rated as to impact insulation class (IIC). The greater the IIC rating, the less impact noise will be transmitted to the area below. Floors may also be rated as to Sound Transmission Class (STC) per ASTM E90. The greater the STC rating, the less airborne sound will be transmitted to the area below. Sound rated floors typically are specified to have an IIC rating of not less than 50 and an STC rating of not less than 50. Even though an IIC rating of 50 meets many building codes, experience has shown that in luxury condominium applications even floor-ceiling systems having an IIC of 56-57 may not be acceptable because some impact noise is still audible.
In addition to having an adequate STC and IIC rating, an acceptable sound rated floor should also have a relatively low profile. Low profile is important to maintain minimum transition height between a finished sound rated floor and adjacent areas, such as carpeted floors, which ordinarily do not need the sound rated construction. Low profile is also important for maintaining door threshold and ceiling height dimensions, restraining construction costs, and maintaining other architectural parameters.
Also, a sound rated floor must exhibit enough vertical stiffness to reduce cracking, creaking, and deflection of the finished covering. At the same time, the sound rated floor must be resilient enough to isolate the impact noise from the area to be protected below. Thus, designers of acoustic flooring must strike a balance between vibration dampening and structural integrity of the floor.
Two isolation media currently used and also approved by the Ceramic Tile Institute for sound rated tile floors are (i) 0.4 inch ENKASONIC® brand matting (nylon and carbon black spinerette extruded 630 g/sq. meter) manufactured by Colbond Inc. of Enka, N.C. and (ii) 0.25 inch Dow ETHAFOAM™ (polyethylene foam 2.7 pcf) manufactured by Dow Chemical Co., Midland Mich. While both of these systems are statically relatively soft and provide some degree of resiliency for impact insulation, the added effect of air stiffness in the 0.25 and 0.40 inch thick media makes the system very stiff dynamically and limits the amount of impact insulation. Because the systems are statically soft, they do not provide a high degree of support for the finished floor, and a relatively thick ( 7/16 inch) glass mesh mortar board, such as a product called Wonderboard, is used on top of the media to provide rigidity for preventing grout, tiles, and other finished flooring from cracking. Alternatively, a relatively thick (1¼ inch) reinforced mortar bed must be installed on top of the resilient mat.
Another known isolation system includes the installation of pads or mounts placed on a subfloor, wooden sleepers are then laid over the isolation pads or mounts, and a plywood deck is fastened to the sleepers to form a secondary subfloor. Often, glass fiber insulation is placed in the cavity defined between the sleepers. A poured or sheet-type underlayment material is then applied to the secondary subfloor. While acoustically effective in reducing sound transmissions, this system adds as much as 6 inches to the thickness of a floor. This thickness is undesirable in most commercial and multi-family residential buildings.
Other known acoustic flooring materials include a poured settable underlayment sold under the mark LEVELROCK™ by United States Gypsum Company of Chicago, Ill. (USG). LEVELROCK underlayment is a mixture of Plaster of Paris, Portland Cement and Crystalline Silica. LEVELROCK underlayments have been used with sound reduction mats (SRM) located between the underlayment and the subfloor. Such mats are made of polymeric material and are typically a matrix of hollow cylindrical shapes held together by a thin mesh. Another material used to dampen sound transmission is Sound Reduction Board (SRB) sold by USG of Chicago, Ill., also under the mark LEVELROCK™. SRB is a mixture of man-made vitreous fiber and minerals, including slag wool fiber, expanded Perlite, starch, cellulose, Kaolin and crystalline silica.